(1) Field of the Invention
The present invention relates to the containment of radioactive materials and particularly to the production of shielding materials suitable for use in the absorption and deceleration of nuclear radiation. More specifically, this invention is directed to novel lead alloys and lead-loaded cements including such lead alloys. The invention also encompasses the manufacture of lead alloys for use in lead-loaded cement and the production of reinforced and unreinforced lead alloy granulate concrete. Accordingly, the general objects of the present invention are to provide novel and improved methods and materials of such character.
(2) Description of the Prior Art
The progressive substitution of nuclear energy for other energy sources, and particularly for combustion processes employing fossil fuels, requires the implementation of stringent safety measures which insured protection from nuclear radiation. Such protection is required, not only in the vicinity of a nuclear reactor, but also in the course of transportation and use of radioactive materials for medical and research purposes and in the transportation and storage of radioactive wastes. Thus, in order to avoid radioactive contamination, walls and enclosures must be provided which are capable of absorbing nuclear radiation, particularly neutrons and gamma rays, or reducing such radiation to a level where its biological activity does not exceed accepted dosage standards.
The materials and screens used for protection against nuclear radiation must be designed by taking into account those forms or types of radiation which have the maximum penetrative power and range; i.e., gamma rays and neutrons. Effective protection against gamma rays requires high-density materials with a high atomic number. The material most commonly employed for shielding sources of gamma radiation is lead.
With regard to neutrons, effective shielding requires that a distinction in the energy spectrum be made between fast neutrons; i.e., those with an energy in excess of approximately 100 ev; and slow or thermal neutrons. Fast neutrons have to undergo thermalization; i.e., they must be slowed down to the thermal neutron energy level; before they can be effectively captured or absorbed. The materials or elements suitable for the slowing down of fast neutrons are those with a low atomic number. The materials which can advantageously be used for the absorption of thermal neutrons are those with a high effective neutron capture cross-section such as, for example, boron.
As is known to those skilled in the art, and as will be obvious from the above brief discussion, efficient protection against nuclear radiation can be provided only through utilization of a material which slows down and absorbs neutrons and which reduces the intensity of gamma rays. However, because of the different characteristics of neutrons and gamma rays, no single material can provide effective shielding with respect to both types of radiation. Restated, in order to effectively shield a source emitting both neutrons and gamma rays, a material consisting of at least two different materials must be used; one material serving for the deceleration and/or absorption of the neutrons and the other for reducing the intensity of the gamma rays.
One material which is known to satisfactorily fulfill the above discussed shielding requirements consists of a lead powder mixed in a synthetic binder such as a polyester or epoxy resin. While the lead containing synthetic resin materials have provided a solution for the problem of affording protection against nuclear radiation, and particularly against neutrons and gamma rays, these resinous materials suffer the drawbacks of comparatively high expense and comparative instability when exposed to nuclear radiation and to heat. Accordingly, shielding materials consisting of lead and a synthetic resin binder clearly are not an ideal choice for most applications.
Considerable research directed to the development of a binder with which lead can be mixed, and which will have economic advantages when compared to the previously proposed synthetic resins, has been conducted. For most applications the most attractive material for use as such an economical binder is hydraulic cement. Thus, over the years attempts have been made to perfect a lead-loaded concrete. Previously proposed lead concretes have, however, proven to be less than satisfactory. As mentioned above, to insure effective attenuation of gamma rays, a material of very high density is required. In the types of lead concrete hitherto foreknown, the lead has been incorporated in the concrete in a pulverous state and the resulting material has had a maximum density of 6.5. This density is insufficient to insure effective protection against gamma radiation. A further deficiency of previously proposed lead concretes resides in the fact that chemical reactions occur between the concrete and the lead. More precisely, when brought into contact with the moisture and water contained in the cement, the lead will oxidize and expand and this expansion will result in fissuring and disintegration of the concrete.